CENTENNIAL, Colo. (Reuters) - A neuropsychologist who ran tests on Colorado movie massacre gunman James Holmes over three days after the 2012 attack told jurors at his murder trial on Thursday there were no signs the shooter was faking mental illness.

Prosecutors say Holmes, 27, is a cold-blooded mass killer who aimed to murder all 400 people watching a midnight premiere of a Batman film at a Denver area cinema, and say he has only pleaded insanity to escape execution.

His court-appointed lawyers say the former neuroscience graduate student, who had never been in trouble before, suffers from schizophrenia, that he heard voices in his head commanding him to kill, and that he was not in control of his actions.

Robert Hanlon, a Chicago-based clinical neuropsychologist, described for the court the tests he performed on Holmes on behalf of the defense team over more than 13 hours of jail house sessions in April 2013.

He said he evaluated the defendant's social cognition, which includes how people behave in social interactions, his brain function, such as problem-solving and reasoning, and his intelligence.

Holmes lawyer Daniel King asked if there were any signs that Holmes was faking psychiatric or psychotic symptoms.

Hanlon also studied Holmes again in January of this year, and said the defendant's I.Q. had declined to 116, from 123 in April 2013, which Hanlon said was "abnormal" and "strongly suggestive of some kind of pathological process."

The psychologist was not asked by the defense to give a diagnosis on Holmes' state of mind at the time of the massacre, only to test his cognitive functions. Also, Hanlon did not study the gunman's behavior in the run-up to the rampage.

On cross-examination, prosecutor George Brauchler said that the so-called "validity" tests used to check whether a subject was malingering were limited.

"They don't give us some sort of guarantee that the defendant is being honest, or accurate, or forthcoming with us in our face-to-face interviews with him, correct?" Brauchler asked.

"True," Hanlon replied.

Under Colorado's insanity defense statute, prosecutors have the burden of proving Holmes was sane.

Two court-appointed psychiatrists have told jurors they concluded that, while the defendant is seriously mentally ill, he was legally sane when he planned and carried out the attack.